Implement nvs save tool

It does not work because esp provisioning and flutter_libserialport
depend on Flutter. We should be able to replace both with pure dart
versions.

dart/shrapnel_test_lib/lib/src/shrapnel_test_lib_base.dart

@@ -266,6 +266,28 @@ Future<void> nvsLoad(String binaryPath, {required String port}) async {
   _log.info(result.stdout);
 }
 
+Future<void> nvsSave(String binaryPath, {required String port}) async {
+  _log.info('Saving NVS partition');
+
+  // pip can be used to install esptool into the global environment:
+  // https://docs.espressif.com/projects/esptool/en/latest/esp32/#quick-start
+  const command = 'esptool.py';
+  final args = '-p $port '
+      '-b 2000000 '
+      '--before default_reset --after hard_reset '
+      '--chip esp32 '
+      'read_flash --flash_mode dio --flash_size 4MB --flash_freq 80m '
+      '0x9000 0x6000 $binaryPath'
+      .split(' ');
+
+  final result = await Process.run(command, args);
+
+  _log.info(result.exitCode);
+  _log.info(result.pid);
+  _log.info(result.stderr);
+  _log.info(result.stdout);
+}
+
 /// UART driver
 class ShrapnelUart {
   ShrapnelUart._(this.port, this.reader) {

dart/shrapnel_test_tool/bin/shrapnel_test_tool.dart

@@ -1,5 +1,5 @@
 import 'package:shrapnel_test_tool/shrapnel_test_tool.dart' as shrapnel_test_tool;
 
 void main(List<String> arguments) {
-  print('Hello world: ${shrapnel_test_tool.calculate()}!');
+  shrapnel_test_tool.main(arguments);
 }

dart/shrapnel_test_tool/lib/shrapnel_test_tool.dart

@@ -1,3 +1,12 @@
-int calculate() {
-  return 6 * 7;
+import 'package:logging/logging.dart';
+import 'package:shrapnel_test_lib/shrapnel_test_lib.dart';
+
+final _log = Logger('shrapnel.test_tool');
+
+void main(List<String> arguments) {
+  hierarchicalLoggingEnabled = true;
+  Logger.root.onRecord.listen(print);
+
+  _log.info('hello $arguments');
+  nvsSave('nvs_${DateTime.now().toIso8601String()}.bin', port: '/dev/ttyUSB0');
 }